This invention relates to magneto-optical (or thermo-magnetic) recording media, and particularly to a magneto-optical recording medium comprising a three-layered magnetic film.
In general, thermo-magnetic (or magneto-optical) recording of information on a recording medium designed for reading the information bits (magnetic domains) through a magneto-optical interaction is carried out as follows. The recording medium, having a magnetic thin film capable of perpendicular magnetization, is preliminarily subjected to a so-called initializing treatment, namely, a treatment for aligning the magnetization directions into one sense perpendicular to the film plane. Thereafter, the magnetic thin film is locally heated by irradiation with a laser beam or other means to form magnetic domains which have a perpendicular magnetization in the opposite sense to the initialized magnetization-direction, whereby information is recorded as two-valued information bits.
In the thermo-magnetic recording method as above, rewriting of information must be preceded by a process for erasing recorded information, which process corresponds to the aforementioned initialization. According to the method, therefore, a certain period of time for erasing is needed prior to rewriting, and it is impossible to record information at a high transfer rate. To overcome this problem, there have been proposed a variety of methods based on the so-called overwrite system which does not need time for such an independent erasing process. Some of the thermo-magnetic recording methods based on the overwrite system are regarded as promising. For example, a method in which a modulated magnetic field is externally applied to the recording medium (external field modulation method) and a method in which an erasing head is used in addition to a recording head (two-head method) have been known.
According to the external field modulation a recording medium comprising an amorphous ferrimagnetic thin film having an easy axis of magnetization perpendicular to the film plane is irradiated with a heating beam, and a magnetic field with a polarity corresponding to the condition of an input digital signal current is applied to the irradiated region of the recording medium, thereby recording.
This type of magneto-optical recording media generally comprise a light-transmitting substrate formed of polycarbonate or the like, a dielectric layer provided on the substrate, a magnetic layer formed on the dielectric layer, and a protective layer or the like provided on the magnetic layer. The recording medium is irradiated with a reading light from the transparent substrate side, so as to read the magnetization of each magnetic domain in the irradiated area by utilizing the Kerr effect. An aluminum (Al) layer may be provided between the magnetic layer and the protective layer, either as a film for reflecting the reading light or in order to enhance the Kerr effect.
However, there has been a tendency not to provide such aluminum (Al) layer in a magneto-optical recording medium having a perpendicular magnetization film as a magnetic layer for magneto-optical recording and designed for reading of recorded information through the Kerr effect on the reflection at the film itself, or in a magneto-optical recording medium in which the thickness of a magnetic layer for magneto-optical recording is large, as in light intensity modulation overwrite type recording media, or the like. The reason is that the aluminum layer, if provided, would produce a heat-radiating effect to thereby lead to a lowered recording density. The magneto-optical recording media which are not provided with such aluminum layer, on the other hand, may suffer the problem of corrosion (pitting) of the magnetic layer due to erosion from the protective layer side.
In order to solve the above-mentioned problems, the present inventors have previously proposed a magneto-optical recording medium in which a protective layer on a magnetic layer for magneto-optical recording, i.e. magneto-optical recording layer, is covered by a resin protective layer, with a 25 to 200 .ANG. thick aluminum thin film interposed therebetween, in Unexamined Japanese Patent Publication HEI 3-86947. One example of the magneto-optical recording medium is shown, in schematic enlarged section, in FIG. 8. As shown in the figure, the magneto-optical recording medium comprises a light-transmitting substrate 1 formed of polycarbonate or the like on which a dielectric layer 2 of Si.sub.3 N.sub.4 or the like, a magneto-optical recording film 13 formed of TbFeCo or the like corresponding to the aforementioned stacked film, and, further, a protective film 16 composed, for instance, of a Si.sub.3 N.sub.4 or other dielectric layer, an aluminum (Al) thin film 17 and a resin protective film 18, e.g. a UV-curable resin layer, are stacked.
Thus, the Al thin film 17 with 25-200 .ANG. thickness is provided between the protective film 16 and the resin protective film 18, so as to obviate the erosion from the side of the resin protective film 18 and to prevent the lowering of S/N, thereby ensuring an enhanced reliability.
Besides, high-speed recording with a high information transfer rate by the aforesaid external field modulation method requires an electromagnet which operates at a frequency of the order of MHz, for example. Such electromagnet is difficult to prepare and, even if prepared, is not suitable for practical use, because of the great power consumption and heat generation thereof.
On the other hand, the two-head method necessitates an extra head and requires the two heads to be spaced apart. These requirements lead to a heavier burden on drive system, a poorer economy, a lower adaptability to mass-production, etc.
In order to overcome the above difficulties, the present inventors have previously proposed a magneto-optical (or thermo-magnetic) recording method which enables rewriting, or overwriting, to be easily accomplished by only switching in a controlled manner the heating temperature for a recording medium in heating the medium by a laser light or the like, as for example in Japanese Patent Application Laid-Open (KOKAI) Nos. 63-52354 (1988) and 63-52355 (1988). According to the magneto-optical (or thermo-magnetic) recording method proposed by the patent applications, as illustrated in FIG. 6, a magneto-optical (or thermo-magnetic) recording medium having a stacked structure of a first and a second rare earth-transition metal magnetic thin film is used, and a condition in which overwrite recording can be performed is obtained as follows. A first thermal condition which is obtained by heating to a first temperature T.sub.1 approximate to or higher than the Curie temperature Tc.sub.1 of a memory layer and not so high as to cause inversion of sublattice magnetization in a recording layer, under a first magnetic field applied externally, and a second thermal condition which is obtained by heating to a second temperature T.sub.2 equal to or higher than the Curie temperature Tc.sub.1 and high enough to cause inversion of the sublattice magnetization in the recording layer, are modulated in a switching manner according the information to be recorded, for example, "0" and "1". In the subsequent cooling process, the senses of sublattice magnetizations in the memory layer are aligned to the senses of sublattice magnetizations in the recording layer due to exchange coupling forces between the memory layer and the recording layer, whereby record bits (magnetic domains) of, for example, "0" and "1" are produced in the memory layer. Furthermore, by a second external magnetic field or by selecting the composition of the recording layer so that the compensation temperature of the recording layer lies between room temperature and the second temperature T.sub.2, it is ensured that the sublattice magnetization in the recording layer can be inverted by only the first magnetic field applied at room temperature.
In this case, no special process (time) for erasing is required, and it is possible to attain a higher transfer rate and to solve the aforementioned problems involved in the two-head system or the external field modulation system.
The present inventors, in their Unexamined Japanese Patent Publications HEI 2-24801 and HEI 2-121103, have made proposals for controlling the domain wall energy density .sigma..sub.w at room temperature of magnetic domain walls generated between magnetic thin films in the thermo-magnetic recording method. That is, the present inventors have proposed a magneto-optical recording medium in which an intermediate layer having either in-plane magnetic anisotropy or slight perpendicular magnetic anisotropy is provided between a memory layer and a recording layer, so as to stabilize the condition in which the interfacial domain walls exist.
FIG. 9 shows, in schematic enlarged section, one example of the magneto-optical recording medium according to Unexamined Japanese Patent Publication HEI 2-24801. As shown in the figure, the magneto-optical recording medium 10 comprises a light-transmitting substrate 1 of polycarbonate or the like on which a dielectric layer 2, a-stacked film 9 of magnetic thin films, and a protective layer 6 composed, for example, of a dielectric layer are stacked in succession, the stacked film 9 being composed of a first magnetic thin film, or memory layer 3, an intermediate layer 4 and a second magnetic thin film, or recording layer 5.
Recording and reproduction on the recording medium 10 are carried out as follows. Recording of information is carried out by heating to first and second temperatures T.sub.1 and T.sub.2, similarly to the recording in the thermo-magnetic (or magneto-optical) recording method according to the above Japanese Patent Application Laid-open (KOKAI) No. 63-52354 (1988). The magnetization states of the above memory layer 3 and recording layer 5, corresponding to temperature T, are schematically indicated by arrows in the memory layer 3 and recording layer 5 in FIG. 7. As shown in the figure, information bits, for example "0" and "1", are recorded in the form of state A wherein the senses of magnetization in the memory layer 3 and the recording layer 5 are the same at room temperature T.sub.R and state B wherein the senses of magnetization are opposite, respectively.
This recording method will now be explained in more detail. First, an area being in the state A,-for example, is irradiated with a laser light, while the intensity of the laser light or the irradiation time is modulated under control according to a recording signal so that the temperature T of the area is raised to a first heating temperature T.sub.1 approximate to or higher than the Curie temperature Tc.sub.1 of the memory layer 3 and not high enough to cause inversion of the magnetization in the recording layer 5 under a desired recording field (external magnetic field) Hex. The heating brings the memory layer 3 into a demagnetized state C. However, when the stacked film is cooled to or below the temperature Tc.sub.1 after the heating is finished, the memory layer 3 comes to exhibit magnetization. The stacked film 9 is so designed that, in this instance, the exchange coupling force between the memory layer 3 and the recording layer 5 predominates, thereby conforming the sense of magnetization of the memory layer 3 to the sense of magnetization of the recording layer 5. That is, the state A is generated, whereby an information bit, for example "0", is recorded.
Another heating is to bring the temperature of an area to a second heating temperature T.sub.2 higher than the above-described temperature T.sub.1 and high enough to cause inversion of the magnetization in the recording layer 5 under the recording magnetic field (external magnetic field) Hex. By such heating, the memory layer 3 loses its magnetization, whereas the magnetization of the recording layer 5 is inverted by the recording field Hex, resulting in state D. When the stacked film 9 is cooled to the temperature Tc.sub.1 after the heating is finished, the exchange coupling force between the memory layer 3 and the recording layer 5 brings the memory layer 3 into state E where the magnetization of the memory layer 3 is opposite to the magnetization in the initial state. At this point, an external sub-field Hsub as an initializing magnetic field or sub-field is applied to the recording layer 5 so as to invert the sense of magnetization in only the recording layer 5, which is designed to have a comparatively low coercive force at or around room temperature T.sub.R. The result is a magnetization state B wherein a magnetic domain wall is present between the memory layer 3 and the recording layer 5; that is, the sense of magnetization in the memory layer 3 is solely inverted, as compared to the magnetization state A, whereby an information bit, for example "1", is recorded.
Thus, in the magneto-optical recording medium having the above-described construction, the bits of information "0" and "1" are recorded in the form of state A and state B, respectively. The magnetization directions (or senses) can be detected through the Kerr rotation upon irradiation with a reading laser light.
Both of the state A and state B are capable of being overwritten by a light intensity modulation system. That is, a given area of the magneto-optical recording medium is capable of being overwritten with any one of the state A and the state B according to the information bits "0" and "1", by the process of selectively heating the area to the temperature T.sub.1 or T.sub.2 for causing the area to pass through at least the state C, in the same manner as described above, irrespective of whether the initial state of the given area is the state A or the state B.
In the magneto-optical recording medium constructed as above, an exchange energy is acting at the interface between the memory layer 3 and the recording layer 5 stacked on each other; therefore, a magnetic domain wall MW is generated in the first state B. The domain wall energy aw is given by the following formula (1): EQU .sigma..sub.w .apprxeq.2((A.sub.1 K.sub.1).sup.1/2 +(A.sub.2 K.sub.2).sup.1/2) (1)
where A.sub.1 and A.sub.2 are the exchange constants, and K.sub.1 and K.sub.2 the perpendicular magnetic anisotropy constants, of the memory layer 3 and the recording layer 5, respectively.
The conditions necessary for overwriting are represented by the following formulas. First, the condition necessary for preventing a transition from state B to state A at room temperature (-20.degree. C. to 60.degree. C.) is given by the following formula (2): EQU Hc.sub.1 &gt;Hw.sub.1 =.sigma..sub.w /2Ms.sub.1 h.sub.1 (2)
Next, the condition of the following formula (3) must be satisfied, in order to prevent a transition from state B to state E. EQU Hc.sub.2 &gt;Hw.sub.2 =.sigma..sub.w /2Ms.sub.2 h.sub.2 (3)
Further, in order that the magnetization of the memory layer 3 in state E may not be inverted by the external sub-field Hsub, the following formula (4) must be satisfied: EQU Hc.sub.1 .+-.Hw.sub.1 &gt;Hsub (4)
where the plus-or-minus sign (.+-.) on the left side is "+" for the case where the memory layer 3 is a rare earth metal-dominant film and the recording layer 5 is a transition metal-dominant film, and "-" for the case where both the memory layer 3 and the recording layer 5 are transition metal-dominant films.
On the other hand, in order to cause a transition from state E to state B, the following formula (5) must be satisfied. EQU Hsub&gt;Hc.sub.2 +Hw.sub.2 =Hc.sub.2 +.sigma.w/2Ms.sub.2 h.sub.2(5)
For a transition from state C to state A, namely, for alignment of the sense of magnetization-of the memory layer 3 into the sense of magnetization of the recording layer 5, under a heating temperature in the vicinity of the Curie temperature Tc.sub.1 of the memory layer 3, the condition of the following formula (6) must be fulfilled. EQU Hw.sub.1 &gt;Hc.sub.1 +Hex (6)
Further, in order for a transition from state C to state E to be inhibited, the condition of the following formula (7) must be satisfied. EQU Hc.sub.2 -Hw.sub.2 &gt;Hex (7)
Moreover, in order that the magnetization of the memory layer 3 may not be inverted by the external sub-field Hsub at the time of the transition from state E to state B, the condition of the following formula (8) must be fulfilled. EQU Hsub&lt;Hc.sub.1 -Hw.sub.1 =Hc.sub.1 -.sigma..sub.w /(2Ms.sub.1 h.sub.1)(8)
In each of the above formulas, Hw.sub.1 and Hw.sub.2 are effective magnetic fields due to the exchange coupling forces as defined in the above formulas (1) and (2); Hc.sub.1 and Hc.sub.2, Ms, and Ms.sub.2, and h.sub.1 and h.sub.2 represent the coercive force, the saturation magnetization and the thickness of the memory layer 3 and the recording layer 5, respectively.
As is clearly understood from the above, it is desirable for the domain wall energy .sigma..sub.w to have a lower value in order to satisfy the above formulas (2) and (3), through actual values of the domain wall energy .sigma..sub.w are considerably high. Also, it is seen from the above formulas (5) and (7) that the external sub-field Hsub increases with an increase in the film thickness h.sub.2 of the recording layer 5.
According to this method, in which information is recorded in terms of magnetization state of the memory layer 3, the magnetostatic coupling between the memory layer 3 and the recording layer 5 through the intermediate layer 4 having in-plane magnetic anisotropy or slight perpendicular magnetic anisotropy reduces the domain wall energy .sigma..sub.w between the memory layer 3 and the recording layer 5. It is therefore possible to reduce the external sub-field Hsub necessary for the transition from state E to state B, namely, for initialization of the recording layer 5, and to reduce the total thickness of the stacked film 9.
In carrying out the two-valued information recording as above, it is necessary to heat selectively to the first temperature T.sub.1 and second temperature T.sub.2 according to the information to be recorded. In the light intensity modulation system, therefore, the temperature distribution in the area heated should be controlled appropriately. The control has been made in the prior art by regulating the irradiating laser power only. With the magneto-optical recording medium, however, there has been the drawback that the tolerance or margin for a high-output-level laser power PH and a low-output-level laser powert PL set for individual recording of each information bit in two-valued information recording is comparatively small.
Meanwhile, as to the light modulation type magneto-optical recording media, a construction has been proposed in which a metal layer is provided on a dielectric protective film covering a magneto-optical recording film on the side opposite to the substrate side, and further a dielectric layer and a UV-curable resin film are stacked on the metal layer [A. Okamuro, et al., MORIS (MO Recording International Symposium), 18-T-13, 199]. This magneto-optical recording medium, as shown in schematic enlarged section in FIG. 8, comprises a light-transmitting substrate 1 on which a dielectric layer 2, a magneto-optical recording film 13, a dielectric layer 6A, a metal layer 7, a dielectric layer 6B and a protective layer 8 are provided in a sequentially stacked form.
In the magneto-optical recording medium, the metal layer 7 formed on the magneto-optical recording film 13, with the dielectric layer 6A interposed therebetween, serves for heat radiation to thereby moderate the temperature variation in the recording film 13 at the time of irradiation with laser light, and to enlarge the margin in setting the power of the irradiating laser light. According to the proposed construction, the thickness of the magneto-optical recording film 13 is 1000 .ANG., the thickness of the metal layer 7 is 200 .ANG., and the thicknesses t.sub.1 and t.sub.2 of the dielectric layers 6A and 6B are 100 .ANG. each, so as to achieve efficient transfer of the heat generated in the recording film 13 by irradiation with laser light to the metal layer 7. As a consequence of the construction, however, there is a possibility that the laser light power necessary for heating to the desired temperature T.sub.1 or T.sub.2 in the two-valued information recording may be increased.
Besides, because this type of thermo-magnetic recording medium is irradiated with laser light from the side of the memory layer 3, unintended observation of the recording layer 5 through the memory layer 3 can occur, according to the film thickness of the memory layer 3. Therefore, it has been necessary to select the thickness of the memory layer 3 so as to enable assured discrimination between the above-described state E and state B.